The proposed research will study the pathway of selenium assimilation in human lymphocytes and in human tissue culture cells. Techniques for the rapid assay of 75-Se incorporation into glutathione peroxidase will be developed. A survey will be carried out to determine the phylogenetic distribution of glutathione peroxidase. Experiments will be carried out to determine if selenium is assimilated into cyclic compounds such as selenobiotin and selenothiamine.